Chamber 04: This chamber reinforces concepts that Chell has already learned: Portals can be used to reach areas otherwise inaccessible, and that cubes belong on buttons. Chell also has the opprotunity to experiment with portals at different surface angles and their gravitational effects.

Chamber 05: Another 'reinforcement' chamber, although Chell begins to develop suspicious thoughts of GLaDOS. GLaDOS informs Chell that she will not be monitored in Test Chamber 05. But towards the end of the chamber, GLaDOS admits that that was a lie for testing purposes, and that she will stop "enhancing the truth."